1. Field of the Invention
The present invention is related to a system for increasing the conversion yield and reducing energy consumption in heterogeneous synthesis reactors, in particular for ammonia synthesis. Such reactors conventionally consist of an external shell, usually formed of a single piece, and of an internal cartridge which contains granular catalyst set in one or more catalyst beds arranged between an external cylindrical wall formed of parts of the cartridge, an internal cylindrical wall formed of parts of an internal transfer pipe for fresh synthesis gas, and a perforated bottom, the upper cross section of each bed being open and fed by the synthesis gas in an axial flow.
2. Description of the Prior Art
It is known that catalytic synthesis reactors under pressure, in particular for catalytic synthesis of ammonia, methanol, higher alcohols, etc., consist of an external shell, usually formed of a single piece, and of an internal cartridge containing catalyst granules of different forms and characteristics arranged in one or more layers (catalyst beds).
To obtain optimal temperatures in the various catalyst beds with exothermic reactions, the synthesis gas that flows through the various catalytic beds is usually cooled between beds, by means of fresh gas injection (quench reactor) or by direct interchange with the inlet cold gas.
Recently, proposals have been made for reactors with radial flow of the gas in the catalytic beds (U.S. Pat. Nos. 3,918,918 and 4,181,701, and European Patent Appln. No. 007743-A1, assigned to Lummus, Topsoe and Kellogg, respectively) or axial-radial (U.S. Pat. Nos. 4,372,920 and 4,405,562, assigned to the present assignee Ammonia Casale) which constitute great progress as compared with axial flow reactors, in particular when considerable amounts of catalyst are involved, by reducing pressure drops through the catalytic beds and, consequently, reducing energy consumption.
Axial gas flow required the use of reactors developed widthwise (low height-diameter ratio of the apparatus) with high costs of the equipment and high energy consumption.
The Ammonia Casale U.S. Pat. Nos. 4,372,920 and 4,405,562 have allowed the design of greatly simplified synthesis reactor cartridges with an internal structure having easy access for maintenance and for loading and unloading the catalyst and, at the same time, ensuring low pressure drops.
According to the above-mentioned patents, each catalyst bed is provided with a basket composed of an external perforated cylindrical wall, an internal perforated cylindrical wall and only one sealed bottom (the upper part of the basket is completely open); an upper part of at least one of said cylindrical walls being unperforated, the upper ends of said two cylindrical walls being on a plane approximately perpendicular to the longitudinal axis of said perforated walls. These walls form a zone where a minor portion of the gas runs through the catalyst bed in a prevalently axial flow, while the remaining major portion of the gas runs in a radial flow through the major part of the catalytic bed located between the perforated parts of the cylindrical walls of the bed.
Accordingly, the gas portion that runs through the bed in a prevalently axial flow is controlled exclusively by the height of the unperforated upper portion of at least one of said walls, said unperforated portion being only a minor part of the total height of the same wall(s).
Reactors whose dimensions correspond to a height-diameter ratio higher than 10 (i.e. diameter-height ratio less than 0.1) are especially suitable for the above-described technique.
In today's world economic situation, great importance is being given to the market of existing plant modernization. Most existing plants for the synthesis process (e.g. ammonia synthesis) employ reactors with axial flow in the catalytic beds (axial reactors) characterized by a low height-diameter ratio of the apparatus, owing to the above-mentioned requirement to minimize the pressure drops in the reactors. These conventional axial flow reactors have a high energy consumption and other drawbacks which are particularly burdensome when the reactors, as for instance those of the "lozenge" type, use only one catalytic bed which is very high, thus causing heavy pressure drops or energy consumption and strong buckling in the lower layers of the catalyst, which are therefore damaged and lose a portion of their activity.